The present invention relates to impregnated diamond drill bits.
In general, there are two basic types of diamond drill bits: surface set bits and impregnated drill bits. A surface set diamond drill bits has a plurality of diamond particles that are set on the surface of the metallic matrix. In the surface set bits, the diamonds are of such a size that there are approximately 30 to 40 diamond particles per carat. In the impregnated diamond drill bits, the diamond particles are physically mixed with the metallic powder so as to be dispersed in the powder. With the impregnated bits, very small grains of diamonds are used, on the order of 40 to 50 mesh in size, i.e. approximately 1,250 particles per carat. With both types of bits, the diamonds are arranged within the crown of the bit. A steel shank is then attached to the crown.
With the impregnated diamond drill bits, as the metal matrix wears away, new diamonds are exposed with such diamonds providing new cutting surfaces. Ideally, the diamonds should be evenly dispersed within the metal matrix so that as the matrix wears, there are always new diamond cutting surfaces being exposed.
In general, both the surface set bits and the impregnated bits are made by placing the diamond particles within a carbon mold along with the various components of the metal matrix. In the case of the surface set bits, the diamonds are first placed into the mold and then the remainder of the mold is filled with the metallic powder. In forming the impregnated bits, the diamond particles and the powder are mixed together and poured into a mold. In order to obtain better distribution of the diamonds, small quantities of the metallic powder and the diamond particles can be mixed and a plurality of such mixtures poured into the mold, a layer at a time, thereby obtaining a better distribution of the diamond particles. After the mold has been filled with the diamonds and the metallic powder so as to form the metal matrix, heat and pressure are applied and silver or another type of solder is provided to infiltrate the powdered metal matrix and effectively hold the diamonds in the powder matrix thereby forming the crown structure. The crown structure is then bonded to a steel shank thus completing the drill bit.
In accordance with the prior art, the composition of the powdered metal matrix used to hold the diamond particles has primarily consisted of titanium carbide and iron. Once fused, the titanium carbide provides a crown having a hardness in the range of approximately 20 to 25 Rockwell C. The iron powder that is employed in the matrix provides for good infiltration between the diamond particles, thereby improving the soldering, or fusing, process.
In accordance with the prior art, it has been necessary for the metal matrix to have a hardness within the range provided by the use of the titanium carbide in order to adequately hold the diamond particles. If a matrix having a lower degree hardness was used, the diamond particles were not adequately retained during employment of the impregnated diamond drill bit. Various metallic compositions for retaining diamond particles within a structure are disclosed by U.S. Pat. Nos.: 2,703,750 to Cotter; 3,464,804 to Kuratomi et al; and 3,879,901 to Caveney.
Although the titanium carbide matrix securely retained the diamond particles, the hardness of the crown structure caused several major disadvantages. Unfortunately, drill bits having such crown structures are too hard for use in many rock formations. A crown structure having a hardness of between 20 to 25 Rockwell C is so abrasive resistent that as the bit is used the matrix does not wear away so as to expose new diamond cutting edges. As a result, the cutting rate of the impregnated diamond drill bit is impeded and the effective life of the bit is significantly diminished.